Data collected by several countries bordering on the Arctic
allow the construction of maps of the lateral sea ice extent
since the beginning of the 20th century. Data from the first
seven decades were derived from ship and aircraft observation
while data for the last three decades come primarily from
satellites. Figure 1
(from Walsh and Chapman, 2000) shows time series of the
areas enclosed by the ice boundary (defined as having 10%
or less of the surface of the ocean covered by ice).

During the first half-century, ice extent in all seasons
remained essentially constant. Beginning about mid-century,
the summer minimum extent began to shrink while the winter
maximum remained unchanged. Starting in about 1975, the
maximum, too, began to shrink. Ice extent in September 2000
(not shown in the plot) is only about 5x106 km2,
the smallest ever observed in this century.

Information about the average thickness of ice before the
advent of nuclear submarines is limited to occasional samples
taken from ships and manned ice camps. They suggest an average
thickness of 3 to 3.5 meters and little change in time.
Declassified submarine sonar observations of the ice draft
taken between 1958 and 1976 corroborate this information.
However, the unclassified cruises of the SCICEX program,
1993199t show a significant, locally variable, thinning
of the ice by 12 meters (Rothrock et al., 2000). It
is likely that most of this thinning occurred in the last
two decades of the century. These findings are supported
by satellite-based passive microwave observations, which
are capable of distinguishing first-year from multi-year
ice, that the area fraction covered by my multi-year ice
has shrunk by 14% between 1978 and 1998 (Johannessen et
al., 1999).

The average thinning of the ice appears to be the result
of both the diminished fraction of multi-year ice and the
relative thinning of all ice categories.

Both the shrinking and thinning of the arctic sea ice cover
appear to be in keeping with the poleward amplification
of the global warming induced by increased greenhouse loading
of the atmosphere and predicted by interactive climate models.
Recent computations (e.g. Vinnikov, 1999) closely duplicate
the observed reduction of the mean annual ice extent. However,
closer inspection reveals a disturbing discrepancy: models
show impacts in winter and observations show ice retreat
in summer. As we expect from basic physical reasoning, the
largest effects of greenhouse warming should be seen in
the absence of solar radiation when thermal infrared radiation
dominates the surface energy balance, i.e. in winter. The
calculations by Vinnikov et al. (1999) and Manabe et al.
(1992) indeed show the largest sea ice signal in winter.
An explanation of this summer/winter discrepancy has not
been offered so far. The absence and presence of sea ice,
and its thickness, depend on very small differences between
large fluxes of energy. Minor changes of the assumptions
about surface albedo, snow cover, cloudiness and cloud radiative
properties, ocean heat flux, and other factors, may have
large effects on the computed ice cover and require a model
precision that remains to be attained.